The development of cobalt phosphide co-catalysts on BiVO₄ photoanodes to improve H₂O₂ production

Photoanodic hydrogen peroxide (H₂O₂) production via water oxidation is limited by low yields and poor selectivity. Herein, four variations of cobalt phosphides, including pristine CoP and Co₂P crystals, and two mixed-phase cobalt phosphides (CoP/Co₂P) with different ratios, were applied as co-catalysts on the BiVO₄ (BVO) photoanode to improve H₂O₂ production. The optimal yield and selectivity were approximately 9.6 µmol‧h^-1‧cm^-2 and 25.2 % at a voltage bias of 1.7 V vs reversible hydrogen electrode (V_RHE) under sunlight illumination, respectively. This performance is approximately 1.8 times that of pristine BVO photoanode. The roles of the Co and P sites were investigated. In particular, the Co site promotes the breaking of one HO bond in water to form OH^• radicals, which is the rate-determining step in H₂O₂ production. The P site plays an important role in the desorption of H₂O₂ formed from the catalyst, which is responsible for the recovery of fresh catalytic sites. Among the four samples, Co₂P exhibited the best performance for H₂O₂ production because it had the highest rate of OH^• formation owing to its improved accumulation property. This study offers a rational design strategy for co-catalysts for photoanodic H₂O₂ production.